You forget to RSVP.

No one really mails anything anymore. But if there's one thing worth paying postage for, it's the RSVP card to a wedding invitation.

Sending it late is bad manners, and not replying at all is even worse.

"It's easy to goof on getting your reply in on time. But leaving a couple in the dark about your intentions with attending/not attending is more stressful to them than one might think," Jacobs told INSIDER. "For couples planning a wedding, so much depends on getting RSVPs on time. So if you forgot to reply, let them know ASAP if you can or cannot be there!"

Her solution: "If you get an invite, make it a practice to RSVP right away (email, text, DM, whatever) and follow up with the real snail mail card."

You ignore the registry.

"Wedding registries are there to pretty much guarantee that you're going to buy a gift that they [the bride and groom] are going to want," Jacobs said.

You can go an extra step and make your run-of-the-mill registry gift more personal.

"Sometimes a bunch of $10 kitchen gadgets don't always seem so impressive but you can personalize them by bundling them together and gifting with a monthly meal prep membership," the planner said. "Get creative and never re-gift! It's totally obvious."

3/

Your invite doesn't give you a plus one, but you ask for one anyway.

Being at the singles table can be fun.
Tiffany Rose/Getty Images for Wedding Paper Divas

Weddings are expensive, and couples don't always have the budget or venue space to invite as many people as they would like.

"Unless your invitation specifically invited you with a plus one, never make the assumption or ask if you can bring a date," Jacobs said. "If you aren't invited with a guest, never complain about it to the couple. Chances are, the couple has thought through their numbers based on a lot of factors such as room capacity, budget, and even the fact that they want to keep things a certain size."

For unattached wedding guests, it's not so bad. In Jacob's words, "Smile and enjoy that singles table!"

"I always work with my couples at Chandelier Events to make sure we offer guests tons of detailed things-to-know and things-to-do information," she said. Still, couples come under a lot of pressure to make their guests happy. "Remember, use GPS, Google and Glamsquad — if you need directions, a dinner reservation, or a blow-out, don't bother the bride!"

5/

You post a photo of the bride's wedding dress on social media — before the groom has seen it.

You talk or stare at your phone during the ceremony.

"Refrain from talking during a ceremony," Jacobs said. "Even if you think you're whispering, they can hear you." This is especially true in an indoor setting, where the acoustics might make your voice louder. To be on the safe side, just don't say anything.

Answering a phone call, or staring at your smartphone is also rude.

"You can live for 30 minutes without an email or social media distraction," the wedding planner said. "Nothing feels worse than seeing a wedding guest they cherish with eyes down on a football game when you are standing at the altar. (True story.)"

8/

You end up in the professional wedding photos... because you're taking photos.

Many couples hire a professional wedding photographer, so their guests don't need to take photos all the time — yet many still do.

"There is nothing worse for a couple than when they get their wedding photos back and they see tons of cell phones, guests in the aisle, or — dare we say it — a guest with a phone or a camera from behind the altar, in view of the money shot," Jacobs said.

"So put your phones away until they say I do. And if you just have to have a snapshot, be discreet."